NATURE: Birds & Bats, A Cosmic B-flat

More wonders about the sounds around us:  Bats could work on a Boy Scout merit badge for tree identification. The Music of Wild Birds has returned -- the book, that is -- to book shelves. And in space, "no one can hear you scream," perhaps, but it's a noisier place than you might think.

It is well known that bats use echolocation - emitting sound waves and processing the returning echoes - to navigate their surroundings in the darkness of night or deep caves.

New research suggests that bats can "see" with these sounds more clearly than previously thought. Scientists in Germany have determined that bats can evaluate the "smoothness" of the acoustic responses and therefore distinguish different kinds of trees. Pine trees, for example, appear "smoother" than, say, oak trees, because of the larger number of weaker echoes coming from small needles vs. larger, less numerous broad leaf echoes.

Their lab experiments are causing scientists to update the previous belief that bats could not make much sense of the chaotic clutter of environmental echoes.

Visit the NY Times Science Desk [fee required] or just read the actual paper's abstract

“Classification of natural textures in echolocation,” by Jan-Eric Grunwald, Sven Schörnich and Lutz Wiegrebe. Through echolocation, a bat can perceive not only the position of an object in the dark; it can also recognize its 3D structure. A tree, however, is a very complex object; it has thousands of reflective surfaces that result in a chaotic acoustic image of the tree. Technically, the acoustic image of an object is its impulse response (IR), i.e., the sum of the reflections recorded when the object is ensonified with an acoustic impulse. The extraction of the acoustic IR from the ultrasonic echo and the detailed IR analysis underlies the bats' extraordinary object-recognition capabilities. Here, a phantom-object playback experiment is developed to demonstrate that the bat Phyllostomus discolor can evaluate a statistical property of chaotic IRs, the IR roughness. The IRs of the phantom objects consisted of up to 4,000 stochastically distributed reflections. It is shown that P. discolor spontaneously classifies echoes generated with these IRs according to IR roughness. This capability enables the bats to evaluate complex natural textures, such as foliage types, in a meaningful manner. The present behavioral results and their simulations in a computer model of the bats' ascending auditory system indicate the involvement of modulation-sensitive neurons in echo analysis.

According to a feature from National Public Radio, the bird lover, composer and artist would listen to the birds while strolling in nearby fields and woods, then return to his New Hampshire home to translate the songs into musical notation. His book contained those transcriptions, with detailed descriptions and watercolor illustrations.

Judy Pelikan (no kidding!) has just re-illustrated and condensed Mathews' work in a newly released book, "The Music of Wild Birds."

Learn more by visiting the NPR feature, which contains Pelikan's on-air interview, plus musical renditions of Mathews' bird song transcriptions, or just browse the new Pelikan book at Amazon.com

So indeed, space is generally a place that is inhospitable to sound, because sound requires atoms to be "jostled," in turn jostling other atoms. When atoms widely separated, as in the typical interplanetary and interstellar voids -- no sound!

But on other planets and stars -- and even in some nebula, under the right circumstances -- space rocks!

For example:

• "Last September researchers using the Chandra X-Ray Observatory detected sound waves blaring from a distant black hole."
• "The grinding of Europa's thick ice sheets would make the surface of that Jovian moon far from library-quiet to an astronaut there."
• "Saturn's fierce 600-mile-per-hour easterlies produce deafening howls."  [Although, presumably, no one is there to be actually "deafened" by them. Which brings up the old question: If a wind howls on Saturn, … Never mind!]
• "Violent solar flares trigger cast acoustic ripples that spread out for tens of thousands of miles across the surface of the sun."

With enough energy being pumped into it, even the near-vacuum of space can be rippled into sound waves. For example, what the Chandra revealed was a super-massive black hole in the Perseus galaxy cluster, 250 million light-years away from us. Just outside the black hole itself sits a disk of hot gas. From its edges swirl twin jets of plasma, a state of matter best thought of as electrically-charged gas. And those jets create bubbles. Big, persistent bubbles. Regular pulses of heated atoms keep the bubbles from collapsing, and "regular pulses" = sound!

In this case, the frequency is too low for human ears to hear, even if we were in the neighborhood, which we wouldn't want anyway. But if we could hear 57 octaves below middle C, this black hole would sound like a B-flat.

Read more in Discover [full version requires subscription]

Explore the discoveries of the Chandra X-Ray Observatory and how Chandra found the cosmic sound

Visit "The Singing Sun" - hear it sing, watch it boil

Read Rockwell's New York Times essay or follow links to items that Rockwell mentions:

• See a February 2004 Vibrations article about the "mysterious hum" that plagues some communities
• Learn more about composer Terry Riley at New Albion or Terry Riley.com
• Browse a book by Steblin (out of print)